The increase in complexity of advanced electronic communication equipment has brought with it the need for improved diagnostic methods and built-in-testability to pinpoint a failed module, circuit board or component.
While many digital circuits are readily capable of generating self-test signals for digital circuitry, there still remains a great need for improving RF/microwave built-in-test methods.
In the past, several different methods have been used to test multi-channel microwave receivers. One method typically has been to implement a noise diode, towards the front end of the receiver, which generates a white noise signal which can be filtered, amplified and mixed with a local oscillator, all of which are commonly found at the front end of a typical microwave receiver.
Another method often employed is to provide an injection of a test source into the IF stage of the receiver.
A third method would be to generate a test signal at the desired microwave frequency.
While these, and similar, methods and designs have enjoyed considerable use in the past, they have several serious drawbacks. First, the noise diode is not able to be modulated, in order to emulate an actual signal, and consequently, cannot perform a comprehensive receiver test and can only test system gain and noise figure. Moreover, the noise diode requires active circuitry to minimize loading affects on the operational signal path, which results in increased power consumption and cost and reduced reliability. Similarly, the injection of a test source into the IF stage of the receiver, while allowing signal modulation, does not test the microwave circuitry; thereby, leaving an important portion of the receiver chain untested. Generating a test signal at the desired microwave frequency using a microwave oscillator requires increased power dissipation and cost of the receiver. Since a microwave oscillator alone does not have the required accuracy, the test signal must be phase-locked or multiplied up from a lower frequency source, all of which add to the expense and power consumption of self-test circuit.
Consequently, there exists a need for improvements in microwave receivers which can perform a comprehensive self-test without requiring increased power consumption and cost.